Determination of Mass Change of Rubber After Hot Air Aging: Testing Methods, Standards, and Applicat

The determination of mass change in rubber after hot air aging is a critical accelerated aging test used to evaluate the thermal stability, oxidation resistance, and long-term durability of vulcanized and thermoplastic rubber materials. This test measures the percentage of weight loss or gain after exposure to elevated temperatures in an air-circulating oven, providing essential data for quality control, material qualification, and product lifetime prediction. In this article, we explain the testing principles, applicable standards such as ISO 188, ASTM D573, and GB/T 3512, sample requirements, procedures, and typical industrial applications.

Technical Overview

Hot air aging, also known as air oven aging or accelerated thermal aging, is one of the most widely performed durability tests in rubber materials science. During service, rubber products are continuously exposed to oxygen, heat, and mechanical stress, which trigger complex chemical reactions including oxidative chain scission, crosslinking, and the volatilization or extraction of low-molecular-weight additives such as plasticizers, antioxidants, and processing oils. These reactions manifest as measurable changes in the mass of the material.

The determination of mass change after hot air aging quantifies these effects. A conditioned test specimen is weighed before and after exposure to a controlled elevated temperature in an air-circulating chamber for a specified duration. The mass change, expressed as a percentage, is calculated from the difference between the final and initial masses relative to the initial mass. A negative value indicates mass loss, typically caused by the evaporation of volatile constituents, while a positive value indicates mass gain, usually attributable to oxidative addition reactions.

This test is governed by several internationally recognized standards, including ISO 188 (Rubber, vulcanized or thermoplastic — Accelerated ageing and heat resistance tests), ASTM D573 (Standard Test Method for Rubber Deterioration in an Air Oven), and GB/T 3512 for hot air accelerated aging and heat resistance testing. Specimen preparation and conditioning generally follow ISO 23529 or ASTM D1349. Accredited third-party laboratories routinely perform this test for material certification, failure analysis, and research and development programs.

Testing Samples

Vulcanized Rubber Sheets and Plates

Standard cured rubber sheets with a thickness of approximately 2 mm are the most common sample form. Sheets are typically cut into rectangular specimens of suitable dimensions to ensure accurate weighing on an analytical balance. The surfaces must be flat, free of defects such as blisters or porosity, and representative of the compounded material.

Dumbbell and Strip Test Pieces

Dumbbell-shaped specimens cut according to ISO 37, ASTM D412, or GB/T 528 can be used when mass change is evaluated alongside changes in tensile strength and elongation at break. This combined approach allows a comprehensive assessment of aging behavior from a single set of specimens, maximizing data value while minimizing material consumption.

Finished Rubber Products and Components

Actual product samples such as seals, gaskets, hoses, bushings, cable sheaths, and O-rings may be tested either whole or as sectioned pieces. When finished products are evaluated, the specimen preparation method, cut surfaces, and original cure state must be documented, as these factors can influence the measured mass change.

Thermoplastic Elastomers

Thermoplastic rubber and elastomer (TPE/TPV) materials can also be assessed using the same hot air aging methodology, provided that the selected aging temperature does not approach the melting or softening point of the polymer phase, which would introduce confounding physical changes.

Testing Items

The core measured parameter in this test is the mass change rate, calculated as the percentage difference between post-aging and initial masses. In practice, this parameter is frequently evaluated together with related aging indicators to build a complete picture of material degradation:

• Mass change rate after aging at specified temperatures (typically 70 °C, 100 °C, 125 °C, 150 °C, or material-specific temperatures) and durations (24 h, 48 h, 72 h, 168 h, or longer)
• Hardness change before and after aging (Shore A or IRHD), per ISO 48 or ASTM D2240
• Tensile strength retention and change rate, per ISO 37 or ASTM D412
• Elongation at break retention and change rate
• Changes in compression set, tear strength, or density where required by product specifications
• Visual assessment of surface conditions such as cracking, blooming, stickiness, or embrittlement

The acceptance criteria for mass change vary widely depending on the polymer family and application. For example, specifications for EPDM and silicone compounds commonly permit only limited mass loss, whereas highly plasticized compounds may show more pronounced changes that must be accounted for in design.

Testing Methods

The test procedure follows a structured sequence designed to ensure repeatability and comparability of results across laboratories:

• Specimen preparation: Samples are cut from cured sheets or products, ensuring clean edges and uniform dimensions. At least three specimens per condition are recommended for statistical validity.
• Conditioning: Specimens are conditioned in a controlled atmosphere, typically 23 ± 2 °C and 50 ± 5 % relative humidity, for a minimum of 16 hours prior to initial weighing, in accordance with ISO 23529 or ASTM D1349.
• Initial weighing: Each specimen is weighed on an analytical balance with a resolution of 0.1 mg or better, and the initial mass is recorded.
• Aging exposure: Specimens are suspended or placed in an air-circulating aging oven, mounted so that they do not contact each other and that all surfaces are freely exposed to circulating air. The oven must maintain temperature uniformity, generally within ±1 °C at the set point, with a defined air exchange rate as specified in ISO 188.
• Common test conditions: Typical combinations include 100 °C × 72 h for general-purpose compounds, 125 °C × 72 h for heat-resistant grades, and 150 °C × 168 h or higher for specialty elastomers such as EPDM, silicone, or fluorocarbon rubber.
• Cooling: After the aging period, specimens are removed and cooled to room temperature, usually in a desiccator, to prevent moisture absorption before reweighing.
• Final weighing: Specimens are reweighed within a specified time window after removal, commonly within 30 minutes to 2 hours, depending on the standard applied.
• Calculation: The mass change rate is calculated as the difference between the final mass and the initial mass, divided by the initial mass, multiplied by 100 %. Results are reported as the median or mean of the specimens along with the aging temperature and duration.

Interpretation of results requires attention to both magnitude and direction of change. Excessive mass loss often signals the depletion of plasticizers or protective additives, which frequently correlates with hardening and loss of elasticity. A significant mass gain, by contrast, may indicate aggressive oxidation, often accompanied by surface degradation. Either trend beyond specification limits suggests reduced service life and warrants formulation or process review.

Applications

Hot air aging with mass change determination is applied across virtually every industry that uses elastomeric materials:

• Automotive: Qualification of engine mounts, seals, hoses, weatherstrips, brake components, and under-hood rubber parts that must withstand sustained high temperatures.
• Wire and cable: Verification of insulation and sheath compounds for heat resistance in accordance with electrical product standards referencing IEC and UL test protocols.
• Aerospace and rail: Evaluation of gaskets, sealing systems, and vibration isolators subjected to demanding thermal environments.
• Pipeline and oil and gas: Assessment of elastomer seals, diaphragms, and O-rings used in valves, compressors, and downhole equipment.
• Consumer goods and footwear: Quality control of sole compounds, elastic components, and household rubber products.
• Medical and pharmaceutical: Screening of elastomeric closures and components for extractable and volatile content changes under accelerated conditions.
• Research and formulation development: Accelerated comparison of antioxidant packages, cure systems, and polymer blends, enabling lifetime estimation through time-temperature extrapolation techniques such as Arrhenius analysis.

FAQ

What causes rubber to lose weight during hot air aging?

Weight loss is primarily caused by the volatilization and evaporation of low-molecular-weight components such as plasticizers, processing oils, waxes, and antioxidants. In some cases, chain scission producing volatile degradation fragments also contributes. The magnitude of loss depends on the compound formulation, aging temperature, and exposure duration.

Why would rubber gain weight after hot air aging?

Mass gain occurs when oxygen chemically combines with the rubber network through oxidation reactions, adding oxygen-containing groups to the polymer chains or crosslinks. This is more common at moderate temperatures in oxygen-reactive polymers and often coincides with hardening and increased crosslink density.

Which aging temperature should be selected for testing?

The temperature is chosen based on the polymer type, the intended service environment, and the relevant product specification. It must be high enough to accelerate degradation within a practical timeframe, yet low enough to avoid unrepresentative failure mechanisms such as melting or rapid depolymerization. Typical values range from 70 °C to 150 °C for general elastomers and up to 250 °C or 300 °C for silicone and fluorocarbon rubbers.

How many specimens are required for a reliable result?

Most standards recommend testing a minimum of three specimens per condition and reporting the median value. Using more specimens improves statistical confidence, particularly for compounds with heterogeneous filler dispersion or those prone to blooming.

Is hot air aging mass change data suitable for lifetime prediction?

Yes, when performed systematically at multiple temperatures and exposure times, the data can support lifetime estimation through Arrhenius-based extrapolation. However, predictions must account for the fact that oven aging in still or circulating air cannot fully replicate service conditions involving mechanical stress, dynamic flexing, or contact with fluids, so results should be validated against real-world performance data.

Can this test be performed on finished products rather than laboratory sheets?

Yes. Testing actual components is common for incoming material verification and failure analysis. The specimen geometry, cut orientation, and original cure history should be documented, since these factors can influence measured mass change and must be considered when comparing results against specification limits established on standard sheets.